In the past decade, intense research has been performed in the area of nanowires and nanopillars. These semiconductor nanostructures can be synthesized with specific material compositions, axial and radial heterostructures, and on large lattice-mismatched substrates, including silicon. Such attributes, combined with their small cross sections, promise new device architectures and pathways to on-chip photonic integration.
An essential component for a viable photonic circuit, for example, is a high-performance nanowire-based laser. Thus, the current state of the art optical microcavity design and fabrication method for nanowires or nanopillars is for laser applications. Researchers have sought to make efficient nanowire-based lasers by a number of methods, including top-down photonic crystal cavities, micro-stadium resonators, plasmonic waveguides, and nanowires that support whispering gallery modes. These designs and methods often involve multiple material deposition and fabrication steps, including precision alignment since nanowires grown by this method also have random placement.
So far, these demonstrations have largely been limited to single nanowires due to lack of control over position and diameter, thus inhibiting low-loss optical cavity design with sufficient material gain, while avoiding highly detrimental surface recombination. In some cases, the resonant cavity and the nanowire active region require separate lithography and processing steps. In other cases, the cavity is simply limited by optical losses at the facets.
The nanowire-laser demonstrations to date are less efficient at trapping light than some embodiments of the current invention, and consequently the threshold power of these may be larger by a factor of 1000 or more. The large threshold power densities and awkward external coupling schemes make them impractical for large-scale integration. Typical reported values of threshold power density range from approximately 100 kW/cm2 to greater than 1 MW/cm2 and require femtosecond pulses. Furthermore, most reported nanowire-based lasers are multimode, which can limit their utility for communications and multiplexing applications. Lasers such as vertical cavity surface emitting lasers and “top-down” photonic-crystal lasers have similar performance to some embodiments of the current invention, but require growth of thick semiconductor films and cannot be integrated on silicon without extensive and low yield process steps such as wafer bonding.
In order to overcome problems in existing devices and methods, including some of the above problems, and to realize a practical nanowire-based laser solution, a high-Q cavity with effective surface passivation is necessary. Of the different possible candidates for high-Q optical cavities, photonic crystal nano-cavity resonators are an attractive choice due to small mode volume, high spontaneous emission coupling factor, and low threshold power.